For a wide variety of applications, chairs are nowadays provided with features which provide enhanced comfort to the person using the chair. For illustration, office-type chairs are commonly utilized in modern working environments to provide an occupant with a level of comfort while performing certain tasks that require a person to be in a seated position for an extended period of time. One common configuration for such a chair includes a mobile chair base assembly to allow the chair to roll across a floor and a pedestal column supporting the superstructure of the chair. The superstructure may include components which enable the user to adjust certain settings of the chair and to facilitate recline or “tilt” of the chair superstructure, including the back and frequently also the seat of the chair. Such a chair configuration allows users to change their sitting position in the chair as desired. Fatigue may be reduced during long sitting periods.
In recent years, chair designs have implemented a feature where a chair back exerts an increasing force onto the seat occupant as a function of recline angle during a rearward reclining movement of the chair back. The chair seat may also tilt in this process or may be displaced otherwise relative to the chair base. To this end, a spring may be provided which is compressed when the chair back reclines. The torque which must be exerted onto the chair back to maintain the chair back at a given recline angle increases as a function of recline angle. Vice versa, the force exerted onto the occupant by the chair back increases.
For enhanced comfort, it is desired that the force applied by the chair back can be adjusted. For illustration, a light-weight user may prefer a configuration which requires less force to be applied onto the chair back to recline it by a given angle. A heavier user may prefer a recline characteristics which requires him to exert a greater force onto the chair back to recline it by the same given angle. The chair may have a tension adjust system which allows the torque which must be exerted onto the chair back in a recline movement as a function of recline angle to be adjusted.
One approach to adjust a tension of a chair back subassembly is to alter an offset bias or pretension of the spring. Alternatively or additionally, a lever arm length may be adjusted. The latter approach also allows the recline characteristics to be altered in a more versatile manner. A chair back subassembly of this type is described in the PCT application PCT/EP2011/003276.
Tension adjust devices generally include an actuation member which can be manipulated by a person sitting on a chair to adjust the tension. The actuation member, which may be coupled to a gripping part for manual actuation by a user, may be rotatable in a first direction for increasing the tension and in an opposite second direction for decreasing the tension. Problems may occur when the energy stored in the energy storage mechanism is suddenly increased while the user still rotates the actuation member to increase the tension. This may occur when the user reclines while still turning the gripping part for increasing the tension. In this case, there may be an undesired back action from the energy storage mechanism, which may cause the gripping part to move rapidly and in an uncontrolled way.
Various approaches have been suggested to lock off the movement of the handle from the energy storage mechanism to prevent such undesired rapid movement of the handle. Many of these approaches rely on frictional forces. For illustration, by using a screw thread with a small pitch for increasing or decreasing a pretension of spring, the friction in the thread may prevent the handle from moving under the action of the spring even when the user reclines during the adjustment process. A drawback of such an approach is that the pitch of the screw thread must be small to reliably lock off the movement of the handle from a sudden increase in energy in the spring. This is turn means that a fairly large number of rotations is required to adjust the spring preload from its minimum to its maximum value or vice versa. Other mechanisms which rely on friction, e.g. self-locking gearings, may significantly add to the complexity, costs and construction space requirements of the adjustment mechanism, which is undesirable.